Hydrocarbon feedstocks such as petroleum crudes, gas oil, etc. are subjected to various processes in order to isolate and separate different fractions of the feedstock. In refinery processes, the feedstock is distilled so as to provide the various valuable fractions, e.g., light hydrocarbons, gasoline, naphtha, kerosene, gas oil, etc. The lower boiling fractions are recovered as an overhead fraction from the distillation and vacuum columns. The intermediate components are recovered as side cuts from the distillation column. The fractions are cooled, condensed, and sent to collecting equipment. No matter what type of petroleum feedstock is used as the charge, the distillation equipment is subjected to the corrosive activity of acids such as H2S, HCl, organic acids, and H2CO3. The problem of corrosion caused by these acid gases as water condenses in the overhead condensing systems of distillation and vacuum columns is well known. The consequent presence of acidic water leads to the undesirable corrosion of metallic equipment, often rapidly.
The general mechanism of this corrosion is an oxidation of metal atoms by aqueous hydrogen ions. The rate of corrosion is directly related to the concentration of aqueous hydrogen ions. A particularly difficult aspect of the problem is that the corrosion occurs above and in the temperature range of the initial condensation of water. The term “initial condensate” as used herein indicates a phase formed when the temperature of the surrounding environment reaches the dew point of water. At this point a mixture of liquid water, hydrocarbon, and vapor may be present. The initial condensate may occur within the distilling unit itself or in subsequent condensers and other equipment. The top temperature of the fractionating column is normally maintained above the dew point of water. The initial aqueous condensate formed contains a high percentage of HCl. The chlorine comes from salts in the crude, and recently the salt content of crude oil (especially Opportunity Crudes) being used in refineries has increased, generating more chlorides. Due to the high concentration of acids dissolved in the water, the pH of the first condensate can be rather low. Thus, as noted, the condensed water can be highly corrosive. It is important that the first condensate is made less corrosive.
Conventionally, highly basic ammonia has been added at various points in the system in an attempt to inhibit the corrosiveness of condensed acidic materials. However, ammonia has not been effective to eliminate corrosion occurring at the initial condensate due to its volatility. In one non-limiting view, ammonia may be ineffective because it does not condense completely enough to neutralize the acidic components of the first condensate.
Amines such as morpholine and methoxy propylamine have been used successfully to control or inhibit corrosion that occurs at the point of initial condensation within or after the distillation unit. Adding amines to the petroleum fractionating system raises the pH of the initial condensate rendering the material substantially less corrosive. The amine inhibitor can be added to the system either in pure form or as an aqueous solution. In some cases, sufficient amounts of amine inhibitors are added to raise the pH of the liquid at the point of initial condensation to above 4.5; in some cases to between 5.5 and 6.5. Other highly basic (pKa>8) amines have been used, including ethylenediamine, monoethanolamine and hexamethylene diamine.
However, the use of these highly basic amines for treating the initial condensate has a problem relating to the resultant hydrochloride salts of these amines which tend to form deposits in distillation columns, column pumparounds, overhead lines, overhead heat exchangers and other parts of the system. These deposits occur after the particular amine has been used for a period of time, sometimes in as little as one or two days. These deposits can cause both fouling and corrosion problems and are particularly problematic in units that do not use a water wash.
Thus, it would be desirable if a method could be devised that neutralizes acid environments in distillation overheads of hydrocarbon processing facilities that minimizes or reduces deposits of hydrochloride and amine salts.